1. Field of the Invention
The invention relates generally to an apparatus for retaining a workpiece on a workpiece support within a semiconductor wafer processing system and, more specifically, to an improved three piece puck and pedestal base assembly for supporting and retaining large diameter (300 mm or more) semiconductor wafers.
2. Description of the Background Art
Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a wafer within a semiconductor wafer process chamber. Electrostatic chucks typically clamp a workpiece (i.e., a semiconductor wafer) by creating an electrostatic attractive force between the wafer and the chuck. A voltage is applied to one or more electrodes in the chuck so as to induce oppositely polarized charges in the wafer and electrodes, respectively. The opposite charges pull the wafer against the chuck, thereby retaining the wafer.
A diameter of 200 mm is an accepted industry standard for semiconductor wafers. In semiconductor wafer processing equipment, electrostatic chucks are commonly used for clamping 200 mm wafers to a pedestal during processing. For example, in a physical vapor deposition (PVD) chamber 400 such as that depicted in FIG. 4, a 200 mm wafer 402 is electrostatically clamped to a pedestal assembly 404 to ensure that the wafer is stationary during processing. To enhance some PVD processes, the pedestal assembly 404 has a thermal transfer element 406 disposed below an electrostatic chuck 412 to regulate the temperature of the wafer to facilitate efficient processing. Increased demand for 200 mm wafers led to improvements in chuck construction and features for processing this size workpiece. This resulted in higher wafer yield, better temperature control during wafer processing and an overall better quality product.
The latest generation of semiconductor wafers have diameters of 300 mm to accommodate fabrication of even more integrated circuit components on a single wafer. Unfortunately, the larger size wafers carry with them their own set of production problems. For example, wafer processing temperatures can reach as high as 500.degree. C. As such, a larger thermal transfer element is required to provide adequate heating of a 300 mm wafer during processing. Additionally, maintaining adequate and uniform thermal conductivity between the thermal transfer element and the backside of the wafer is essential. One-piece thermal transfer element/chuck units are adequate for processing of 200 mm wafers, but may not be suitable for processing their larger successors. When such one-piece units fail or the ceramic chuck portion cracks from fatigue or excessive thermal expansion, the entire unit must be replaced. This type of repair is usually expensive and time consuming.
One solution is to develop a two-piece assembly whereby the chuck and thermal transfer element are individual components. In two piece assemblies, the chuck portion resembles a disk-like portion and is commonly referred to as a puck. Usually the puck and thermal transfer element are fabricated from different materials (i.e., a ceramic puck and a stainless steel thermal transfer element). As such, these materials are joined together by brazing. Under wafer processing conditions, the braze joint often releases contaminants into a process chamber (a condition known as outgassing) which is undesirable. Additionally, at extreme operating temperatures, differential thermal expansion of pedestal components occur. Specifically, a ceramic puck will expand only half as much as a similarly sized stainless steel (SST) part. The portion of the deposition ring (or similar type device) that contacts the expanding ceramic will tend to grind or scratch the ceramic thereby releasing particles into the chamber. These particles can contaminate a wafer under process, equipment within the chamber, or be carried to other chambers or wafers. As such, defective units rise disproportionately to the total number of produced units.
Therefore, there is a need in the art for an improved two-piece puck and thermal transfer element assembly and an apparatus for joining the puck to a thermal transfer element assembly. Such devices are necessary to improve temperature uniformity across a wafer and reduce maintenance and manufacturing costs of same.